lightbox: Use `ZulipIcons.copy` for the "Copy link" button, not `Icons.copy`

[chrisbobbe]

We've already made available a Zulip-specific "copy" icon, from the Figma, as ZulipIcons.copy. We use it for the "Copy message text" button in the message action sheet.

This issue is to use that icon for the "Copy link" button in the lightbox too, instead of the generic Icons.copy.

Please post before-and-after screenshots in your PR description.

[sarafarajnasardi]

I would like to work on this issue...can you assign it to me?

[cybernova2]

Hello! I'd like to take up this issue.

I'll update the Copy link button in the lightbox to use the Zulip-specific ZulipIcons.copy icon and replace the generic Icons.copy. I'll also add before-and-after screenshots in the PR as requested.

Please let me know if there are any additional expectations. Thanks!